Enhancement and recovery of magnetic exchange coupling properties in SrFe11AlO19@NiFe2O4 core-shell structure by multiple TiO2 and SiO2 nanolayer shells

Abstract

The exchange coupling of magnetic hard/soft SrFe11AlO19@NiFe2O4 (SFAO@NFO) core-shell structure was investigated by taking into consideration the effects of TiO2 (TO) and SiO2 (SO) nanoparticles as multiple shells. The SFAO@NFO core-shell structure was synthesized via an auto-combustion sol-gel method. The NFO nanolayer caused the resulting saturation magnetization, magnetic remanence and coercivity of SFAO to be reduced from 43.18 emu/g, 24 emu/g and 5500 Oe to 17.13 emu/g, 7.74 emu/g and 2300 Oe, respectively. This was attributed to the NFO performance as a magnetic dead layer (MDL), and to the SFAO@NFO exchange coupling effect. The TO and SO nanolayers were also synthesized via a templating approach and a modified Stöber method, respectively, in order to act as additional shells and capping layers, recovering magnetic properties of the initial core-shell structure. It was found that the TO nanolayer shell can serve as a room-temperature ferromagnetic material due to its surface defects and oxygen vacancies. The TO and SO nanolayer shells increased the saturation magnetization, and magnetic remanence of SFAO@NFO from 17.13 to 30.51 and 34.10 emu/g, and 7.74 to 11.57 and 14.06 emu/g, respectively, while also decreasing the coercivity from 2300 Oe to 600 and 850 Oe, respectively. As a result, the additional shells enhanced magnetic exchange coupling properties of SFAO@NFO core-shell structure by removing the effect of the MDL.